The present invention is a method for preparing in situ reinforced silicone elastomers and the compositions prepared by the method. The method comprises forming a mixture comprising an end-functional diorganopolysiloxane; a stoichiometric excess of a hydrolyzable siloxane having hydrolyzable functionality described by formula --R.sup.2 SiQ.sub.3-b R.sub.b, were each R is independently selected from a group consisting of alkyls comprising one to six carbon atoms, alkenyls comprising two to six carbon atoms, and aryls, R.sup.2 is a divalent hydrocarbon radical comprising one to 12 carbon atoms, each Q is independently selected from a group consisting of hydroxy, acyloxys comprising one to six carbon atoms, alkoxys comprising one to six carbon atoms, and substituted alkoxys comprising one to six carbon atoms, and b=0 to 2; and a tin salt catalyst and contacting the mixture with water to effect curing. The resulting silicone elastomers are in situ reinforced to provide high strength.
It is known that crosslinked diorganopolysiloxanes must be reinforced with suitable fillers to obtain silicone elastomers with desirable mechanical properties such as high tensile strength and elongation. A typical filler used for this purpose is pyrogenic silica. The manufacture of pyrogenic silica for this purpose is an energy intensive process resulting in costs that are reflected in the cost of the resulting silicone elastomer. In addition, mixing of silica particles into viscous polymers, such as for the preparation of high-consistency elastomers, requires significant energy and time and gives products with properties which are highly dependent on the mixing process.
In the present method the reinforcing filler is generated in situ, thus avoiding the expense of externally forming the reinforcing filler and the expense, time, and variability associated with mixing the reinforcing filler into the diorganopolysiloxanes. Use of the hydrolyzable siloxane compositions described herein to in situ reinforce silicone elastomers provides a method of controlling the tensile strength, elongation, and durometer properties of the elastomer without the separate addition of reinforcing filler.
Mark et al., Makromol. Chem. Rapid Commun. 3:681-685 (1982), describe a two-step process where silanol end-blocked polydimethylsiloxane (PDMS) or vinyl end-blocked PDMS are end-linked to form elastomeric matrices. The end-linked elastomeric matrices are then swollen with tetraethoxysilane (TEOS) and the absorbed TEOS hydrolyzed in place by immersion of the sample into glacial acetic acid.
Jaing et al., Colloid & Polymer Sci. 262:758-760 (1984), used the same two-step process as described by Mark et al., supra, but rather than hydrolyzing the TEOS in acetic acid used a constant relative humidity chamber to cure the TEOS in situ.
Subsequently, Mark et al., Macromolecules 17:2613-2616 (1984), described a one-step or simultaneous curing in an in situ filling process. In the process, silanol end-blocked PDMS, TEOS, and tin catalyst were mixed at various TEOS to PDMS ratios and cured using atmospheric moisture. The tin catalyst used were dibutyltin diacetate and stannous (II) ethylhexanoate.
Tang et al., Polymer Engineering and Science 25:29-31 (1985), describe a one-step simultaneous curing and filling method where a bimodal mixture of silanol end-terminated PDMS including one long and one short chain polymer fraction was combined with stannous(II) ethylhexanoate and various amounts of TEOS and cured using atmospheric moisture.
Ning et al., Polymer Bulletin 13:155-161 (1985), reported using trifunctional silanes to form resinous reinforcing phases in a two-step process. In this work, a vinyl-terminated PDMS was tetrafunctionally end-linked and then swollen with various amounts of TEOS, ethyltriethoxysilane, or diethyldiethoxysilane and combination of these alkoxysilanes. Precipitation of the alkoxysilanes to form the silica or silicate resin was done by immersion of test samples into an aqueous solution of ethyl amine.
Mark et al., Polymer Bulletin 14:325-329 (1985), report using trifunctional silanes as precursors to form trifunctional silicate resins in a simultaneous one-step curing and filling process. Mark et al. report mixing silanol end-terminated PDMS and stannous(II) ethylhexanoate with either vinyltriethoxysilane, methyltriethoxysilane, or phenyltriethoxysilane at various alkoxysilane to PDMS ratios and curing samples under atmospheric conditions.
Sur et al., Makromol. Chem. 187:2861-2866 (1986), report a two-step process where silanol-terminated PDMS is end-linked and then swollen with either tetramethoxysilane, TEOS, tetrapropoxysilane, or tetrabutoxysilane. The samples were cured by immersion in aqueous solutions containing one of a variety of amine catalysts.
Lampe, U.S. Patent No. 4,341,842, issued Jul. 27, 1982, reports a silicone rubber composition having (A) 100 parts of a silanol end-stopped diorganopolysiloxane polymer with a viscosity varying from 100 to 500,000 centipoise at 25.degree. C., (B) from 0.1 to 15 parts by weight of an alkyl silicate or partial hydrolysis product of the silicate, and (C) from 0.01 to 5 parts by weight of a metal salt of a carboxylic acid, where the composition is cured at room temperature to a silicone elastomer.